1. Technical Field
This disclosure relates to a piezoelectric actuator, a liquid-drop ejecting head including the piezoelectric actuator, and a liquid-drop ejecting apparatus including the liquid drop ejecting head.
2. Description of the Background
Recently, piezoelectric actuators including piezoelectric bodies have come to be used to drive micro devices. There continues to be demand for downsizing such actuators used for micro devices, and likewise the downsizing of such piezoelectric actuators is required.
Conventionally, a multilayer piezoelectric element in which a piezoelectric material is sandwiched between a plurality of electrodes is widely used in a piezoelectric actuator. Although such a multilayer piezoelectric element can provide a large amount of deformation, which is generally desirable, such a multilayer structure may pose a disadvantage in terms of downsizing of a micro device. Further, producing a multilayer piezoelectric element may require high-level processing technologies such as cutting the bulk of piezoelectric elements. Compared to such a multilayer piezoelectric element, a piezoelectric element made of thin film PZT (lead zirconate titanate) has an advantage in terms of downsizing. For this reason, some types of piezoelectric actuators use thin film PZT.
FIGS. 1A to 1C are schematic views illustrating a conventional piezoelectric actuator 510 using thin film PZT. FIG. 1A is a perspective view illustrating a schematic configuration of the conventional piezoelectric actuator 510. FIG. 1B is a sectional view illustrating the piezoelectric actuator 510 cut along a line A-A. FIG. 1C is a sectional view illustrating the piezoelectric actuator 510, which is cut along the line A-A, observed when a diaphragm 511 of the piezoelectric actuator 510 is displaced.
The piezoelectric actuator 510 includes the diaphragm 511 and a piezoelectric element 512 formed of thin film PZT. The piezoelectric element 512 is formed on one face of the diaphragm 511. A first electrode 513 is formed between the diaphragm 511 and the piezoelectric element 512. A second electrode 514 is formed on a face of the piezoelectric element 512 opposite a face on which the piezoelectric element 512 contacts the diaphragm 511. The first electrode 513 and the second electrode 514 are supplied with voltages as illustrated in FIG. 2, illustrating examples of voltages supplied to the electrodes 513 and 514. In FIG. 2, the electrode 513 is supplied with a ground potential while the electrode 514 is supplied with a driving potential Vp for driving the piezoelectric element 512.
In the piezoelectric actuator 510, the piezoelectric element 512 is formed at a middle portion of the diaphragm 511. As illustrated in FIG. 1C, when a driving voltage is supplied to the electrode 514, the piezoelectric element 512 extends and contracts the opposed faces of the diaphragm 511 on which the piezoelectric element 512 is formed. Such a configuration allows the piezoelectric actuator 510 to obtain a large displacement amount in the out-of-plane direction of the diaphragm 511 even with the deformation of the piezoelectric element 512 itself is at a low level.
In the piezoelectric actuator 510 illustrated in FIGS. 1A to 1C, forming the piezoelectric element 512 at the middle portion of the diaphragm 511 can provide a larger displacement amount in the out-of-plane direction of the diaphragm 511. However, for example, if the piezoelectric element 512 is provided over a whole area of one face of the diaphragm 511, the displacement amount in the out-of-plane direction of the diaphragm 511 may be reduced. This is because the whole area of one face of the diaphragm 511 extends and contracts in response to the extension and contraction of the piezoelectric element 512 to prevent deformation of the diaphragm 511. Thus, the higher the degree of integration of the piezoelectric actuator 510, the smaller the width of the diaphragm 511, and the less the diaphragm 511 is deformed.
In such a state, to obtain a larger displacement amount in the out-of-plane direction requires supplying a larger driving voltage to the piezoelectric element 512. However, the larger the driving voltage supplied to the piezoelectric element 512, the more likely the piezoelectric element 512 is to receive damage due to ion migration. Such ion migration may be caused by electrode metal ionized and eluted when moisture in air causes an electrochemical reaction. In particular, the pace of ion migration tends to increase at temperatures of 100° C. or less, current densities of 1 mA/cm2 or less, and/or relatively high humidity. The higher the electric-field intensity, the shorter the breaking time of ion migration. As a result, without any measures take to prevent the effect of humid air, electronic components supplied with high voltage might more easily fail. Further, when Nox, NH3, and Cl in the air are adhered to drops, ion migration is accelerated. Therefore, if an electronic component remains exposed to air, oxidization, salination, and sulfuration may easily arise in the electronic component, resulting in ion migration. Thus, ion migration more easily arises in the piezoelectric actuator 510 supplied with high driving voltage.
Hence, various attempts have been made to enhance the degree of integration and increase the displacement amount in the out-of-plane direction of the diaphragm without increasing the driving voltage.
For example, in one conventional approach, to increase the displacement amount of a diaphragm, a piezoelectric element is divided in the short direction of the diaphragm so that the extension and contraction of the diaphragm in the in-plane direction are opposite the extension and contraction in the out-of-plane direction. However, in the above-described approach, each actuator requires two individual electrodes in addition to a common electrode. Accordingly, the number of components, such as driving drivers, may increase, resulting in increased cost.
In another conventional approach, the electrode is divided into a plurality of pieces in each piezoelectric actuator, and the divided pieces are connected between a plurality of piezoelectric actuators to form a common electrode. However, with this configuration two pulses different in timing are required to increase the displacement amount of the diaphragm, which is not conducive to high-speed driving.
Further, a conventional piezoelectric actuator is known that displaces a diaphragm having fixed ends in the short direction by supplying voltages to piezoelectric bodies disposed between opposing electrodes. In such a conventional piezoelectric actuator, for example, separate opposed electrodes are disposed at a middle portion and peripheral portions of a diaphragm in a cross-section of the diaphragm in the short direction. Further, voltages of different polarities are supplied to the piezoelectric bodies so that the extension and contraction of the piezoelectric bodies become opposite between the middle portion and the neighboring portion.
However, in the above-described conventional piezoelectric actuator, since the piezoelectric bodies are provided all over one face of the diaphragm, the piezoelectric bodies disposed at areas in which the opposing electrodes are not provided may reduce the displacement of the diaphragm, effectively preventing an increase in the displacement amount of the diaphragm.
In another conventional piezoelectric actuator, piezoelectric bodies and electrodes on one face of the respective piezoelectric bodies are separately disposed at middle and peripheral portions in the cross-section of a diaphragm in the short direction. However, although the piezoelectric bodies are provided at the middle and peripheral portions of the diaphragm, the displacement of the diaphragm may be reduced depending on the positions of the piezoelectric bodies, effectively preventing any increase in the displacement amount of the diaphragm.